1. Field of the Invention
The present invention relates to a signal distribution system, transmission device, reception device, signal distribution method, transmission method, reception method, medium and information assembly which are, for example, utilized for an entertainment system in an airplane, or the like.
2. Related Art of the Invention
In recent years entertainment services such as viewing movies, listening to music, playing games, in-flight shopping, telephone calls and contacting flight attendants are provided through the terminal installed on the back of the seat or in the armrest part in an airplane and the enhancement of the performance of these services is advancing. In order to implement these services an SEB (seat electric box) that can be controlled by each individual is given to in-flight passengers. In this SEB one from among a plurality of video signals and audio signals sent from a video and audio transmission device installed at the front of the cabin of the airplane can be selected.
This plurality of signals are, respectively, analog modulated or base band modulated and, after that, are frequency multiplexed in one coaxial cable so as to be transmitted to the SEB of each passenger via an ADB (area distribution box). At this time, the boxes are positioned in separate places from each other and, therefore, the total cable length from the transmission device to the SEBs of the passengers sometimes becomes from one hundred to several hundreds of feet.
In such a system, an optimal signal level must be supplied to the SEB of each passenger taking into consideration the transmission loss that occurs in the cable. This is because when the signal level is too low, the video signal becomes degraded and noise (snow) appears on the screen while, contrarily, when the signal level is too high interference is caused in neighboring channels.
Furthermore, since the frequency range transmitted through a coaxial cable is usually in the range of from several tens of MHz to the vicinity 300 MHz, attention must be paid to the loss due to high frequency. This is because the loss at high frequencies is considerably large in comparison with the case where a low frequency is used. In addition, when the total cable length is long a tilt sometimes appears in the frequency characteristics.
Here, the configuration and operation of an in-flight entertainment system according to a prior art are concretely described in reference to FIG. 6. Here, FIG. 6 is a configuration diagram of an in-flight entertainment system according to a prior art.
In FIG. 6 video signals and audio signals inputted from a plurality of input sources are supplied to a PESC (passenger entertainment service controller) 60. The inputted signals are processed in a processing circuit 61 in this PESC 60 and so as to be supplied to a variable gain amplifier 62. This amplifier 62 has an output to a coaxial cable and supplies a signal to an ADB 70.
At the ADB 70 the inputted signal is transferred to another ADB via a line 71 and, at the same time, is sampled in a tap division circuit 72 so as to be supplied to a variable equalizer 73. A signal from this variable equalizer 73 is supplied to a variable gain amplifier 74. The output of the variable gain amplifier 74 is supplied to an SEB 80 via a coaxial cable 75.
The SEB 80 is provided below a passenger seat and includes a tap division circuit 81 wherein this circuit 81 divides the signal and transfers the signal to another SEB on the cable 82 while, on the other hand, samples a signal and gives this signal to the input of a variable gain amplifier 83. In addition, the amplifier 83 supplies the signal to a tuner 84 provided in a passenger seat.
Here, in order to guarantee the RF level wherein the turner 84 is operated under optimal conditions, the PESC 60, the ADB 70 and the SEB 80 are provided, respectively, with a microprocessor 63, a microprocessor 76 and a microprocessor 85 which monitor the reception level of the tuner and the output level of the variable gain amplifier and control the variable equalizer 73, the variable gain amplifier 62, the variable gain amplifier 74 and the variable gain amplifier 83 so that the reception level of the tuner provided in a passenger seat becomes optimized (an associated technology is, for example, disclosed in Japanese Patent Laid-Open H5(1993)-207321).
In the configuration of such an in-flight entertainment system according to the prior art, however, a plurality of variable gain amplifiers and variable equalizers are controlled by using microprocessors and, therefore, the configuration and processing in the device become complicated and it is difficult to effectively transmit data.
For example, in the control of a variable equalizer, unmodified carrier wave signals of a pair of separate frequencies (usually the highest frequency and the lowest frequency) are utilized in the utilized frequency range so as to carry out an adjustment of the tilt which occurs in a cable based on the reception level difference of a tuner. Accordingly, in the case that there is an abnormality in the unmodified carrier wave signals (for example, a fault or degradation in the unmodified carrier wave generation circuit), a mistaken equalizer characteristic is set and, in order to avoid this, processing tends to become complicated.
In addition, a conventional modulation system utilized in a cable transmission is an analog modulation (for example, vestigial side band modulation, or the like) and is not a digital modulation (for example, quadrature amplitude modulation, or the like). In the case of a digital modulation system even in the case that the reception level of the tuner is low, there is almost no effect given to the video quality by combining error corrections and it is acceptable as long as the reception level is within the range of the reception limit while in the case of analog modulation it is necessary to finely adjust the reception level of a tuner and the configuration of a device tends to be complicated.
In this manner, there is a problem in the in-flight entertainment system according to the prior art that data cannot be effectively transmitted.